A radio (wireless) band of Sub-GHz band including 900 MHz band or 2.4 GHz band can provide inexpensive radio communication devices which perform radio communication in which relatively wide areas can be covered. For this reason, it has been studied that radio communication devices capable of performing radio communication in the radio band of Sub-GHz band or 2.4 GHz band are incorporated into apparatuses or equipment.
For example, it is considered that such radio communication devices are incorporated into home electric appliances such as air conditioners and laundry machines for household use and thereby home networks are constructed, to provide advantages such as power saving, improved convenience, monitoring of failures, etc. In the electric home appliance home networks, communication can be performed between each electric home appliance and an access point attached in the vicinity of a pull-in position of Internet.
In recent years, attention has been paid to a smart meter which attains a higher function by adding a communication function to an electric power meter, a gas meter, a tap water meter, etc. Using the smart meter, communication can be performed between a radio communication device attached to a meter at home and an access point provided outdoors, for example, on an electric pole or on a building.
In systems such as the above stated home electric appliance home network, there is a problem that each radio communication device is provided in a fixed position and cannot be transported easily, or a radio wave propagation path becomes a multi-path environment due to presence of many obstacles such as walls or floors, and a reception level significantly degrades in a localized area due to fading (see FIG. 31). The reception level is a value decided based on an intensity (electric field intensity) of a radio wave signal (RF signal) being received. Whether or not the reception level is good is decided on the basis of a reception sensitivity, i.e., a minimum radio wave intensity required to ensure a reception quality necessary for the communication.
FIG. 31 shows a transmitter provided in a multi-path environment and a fluctuation (change) in a reception electric field level in a receiver provided (on a circumference) at an equal distance from the transmitter.
The reception level is significantly different depending on a location in the case where the receiver is provided on the location (localized area) on the circumference at an equal distance from the transmitter. This is due to the fact that a so-called fading phenomenon occurs, in which electric field levels are cancelled from each other or summed up, due to multi-paths of the radio waves. As can be seen from a cumulative probability distribution graph in FIG. 32, there exist an area in which the reception level is lower than a median value of the reception level by 8 dB or higher occupies 10% of the total, and an area in which the reception level is lower than the median value of the reception level by 18 dB or higher occupies 1% of the total.
For example, in the case of a radio (wireless) terminal used in mobile communication, such as a cellular phone, a communication environment changes as it moves. Therefore, a possibility that its reception level continues to be lowered in a localized area for a long period of time is low.
By comparison, the radio communication device incorporated into an electric home appliance or a radio communication device attached to a meter does not move, and therefore, its communication environment changes less. Therefore, there is a possibility that the reception level continues to be low for a long period of time. In this case, if the radio communication device fails to receive a communication packet, restoration from a discommunication state is difficult even though the communication packet is re-transmitted (transmitted subsequently). This must be addressed by using a diversity antenna.
Regarding the diversity antenna, there are a selective diversity method in which an antenna with a better reception level is selected from two antennas and a maximum ratio composition diversity method in which signals received by two antennas are input to two demodulation circuits, respectively, and two demodulated signals are composited optimally such that their phases conform to each other and their amplitudes conform to each other (e.g., see Patent Literatures 1˜5).
Typically, in the case of using the selective diversity method, the lowering of the reception level of the cumulative probability 1% in a location can be improved by about 5 dB.    Patent Literature 1: Japanese Laid-Open Patent Application Publication No. Hei. 4-304036    Patent Literature 2: Japanese Laid-Open Patent Application Publication No. Hei. 5-284435    Patent Literature 3: Japanese Laid-Open Patent Application Publication No. Hei 8-8633    Patent Literature 4: Japanese Examined Patent Application Publication No. Sho. 59-6536    Patent Literature 5: International Publication No. 2006/103758